Chemistry 2025 HSC exam pack

Students should:

• read the question carefully to ensure that they do not miss important components of the question
• have a clear understanding of key words in the question and recognise the intent of the question and its requirements
• plan the response to assist in the logical sequencing of information
• integrate relevant scientific terms into their responses
• engage with any stimulus material provided and refer to it in their response
• show all working in calculations and include correct units and significant figures
• present a logical and succinct response that addresses the question
• review their response to ensure that it addresses the question requirements.

Question 21

In better responses, students were able to:

• use IUPAC nomenclature to name the product
• identify the conditions required for a substitution reaction with organic molecules
• correctly interpret structural organic formula.

Areas for students to improve include:

• knowing the naming conventions for organic molecules and structures
• stating reaction condition(s) and not the type of reaction.

Question 22

In better responses, students were able to:

• identify that quantitative analysis measures amount of the substance that is present and qualitative analysis identifies substances
• provide relevant examples of substances that are assessed quantitatively and qualitatively.

Areas for students to improve include:

• recognising the difference between qualitative and quantitative analysis
• relating analysis to chemical concepts, for example, specific ions and their role in water quality
• identifying techniques used to determine concentration or presence of ions.

Question 23

In better responses, students were able to:

• provide two clear and valid changes to the procedure with justification for improved accuracy
• state how changes to the procedure would improve accuracy for determining percentage composition of sulfate in the fertiliser.

Areas for students to improve include:

• recognising the difference between accuracy and reliability in experiments
• reading the given procedure carefully and not suggesting unnecessary changes
• reviewing experimental skills, such as drying to a constant mass during gravimetric analysis.

Question 24 (a)

In better responses, students were able to:

• draw the structure showing all bonds correctly and the correct number of H and C atoms.
• name the structure correctly based on shape.

Areas for students to improve include:

• understanding the difference between the structural formula and the shape of molecule
• reading the question carefully to determine which compound to draw.

Question 24 (b)

In better responses, students were able to

• apply chemistry of addition reactions, rather than substitution
• provide structures or formulae for the chemicals, rather than repeat the provided chemical names
• use given molar masses to determine the moles and then the mass of the substance.

Areas for students to improve include:

• writing balanced chemical equations
• writing chemical formulae with the correct number of bonds, for example, 4 per carbon
• not using equilibrium arrows, given the assumption that a limiting reagent was completely consumed
• using values that are supplied in the question.

Question 25

In better responses, students were able to:

• understand the stimulus information and how to use density and volume or density and molar mass to perform a calculation
• employ correct significant figures throughout and at the end of their working
• apply % yield, rather than % error or % atom economy.

Areas for students to improve include:

• applying accurate significant figures to all questions
• recognising stimulus information to perform accurate calculations, for example, density and moles
• understanding the definition of yield.

Question 26 (a)

In better responses, students were able to:

• identify the independent variable consisted of categorical data
• construct a column graph with an accurate scale and clear axis labels
• plot points and column heights accurately.

Areas for students to improve include:

• understanding that line graphs are appropriate only when the independent and dependent variables contain quantitative, continuous data
• taking care to plot points and columns accurately and clearly
• ensuring the axis of the graph has an appropriate scale
• creating scales that fit the data to be graphed.

Question 26 (b)

In better responses, students were able to:

• identify that the strength of the haloethanoic acid decreases as the molar mass of the halogen increases / electronegativity decreases
• identify and describe the relationship between pK_a and K_a, for example, as pK_a increases, K_a decreases and therefore acid strength decreases as the halogens in the haloalkanoic acids go down the group
• identify and express a relevant trend in the halogen series.

Areas for students to improve include:

• describing a trend in the given data, rather than reiterating the data given in the question, for example, fluorine has a pK_a of 2.6 while iodine has a pK_a of 3.2
• recognising the relationship between pK_a and acid strength
• recognising electronegativity decreases down group 7
• understanding -log scales and the resulting inverse relationship.

Question 27 (a)

In better responses, students were able to:

• select one hydrocarbon mixture from the list provided
• understand that mixtures such as petrol contain several hydrocarbons
• provide a clear and specific use for the named mixture
• outline a clear environmental implication arising from the use of the mixture.

Areas for students to improve include:

• identifying the mixture, rather than naming a single compound
• developing a clear link between the identified use and its environmental impact
• ensuring environmental impacts are described clearly and specifically, for example, distinguishing between 'greenhouse gases' and the 'greenhouse effect'
• understanding the difference between the greenhouse effect and ozone depletion.

Question 27 (b)

In better responses, students were able to:

• provide a correct chemical equation using full structural formulae for both reactants and products
• represent the functional group in both the organic reactants and products accurately
• include all relevant reaction conditions above the arrow, identifying a specific acid used as a catalyst and dilute conditions
• show a clear understanding of the addition reaction process.

Areas for students to improve include:

• ensuring the correct type of formula is used, for example, condensed versus structural
• demonstrating the C–O–H arrangement of the hydroxyl group
• adding all hydrogen atoms when drawing structural formulae
• identifying and placing reaction conditions directly above the arrow.

Question 27 (c)

In better responses, students were able to:

• show the immediate vertical increase at t₁ due to the sudden addition of ethanol
• represent the decrease in ethanol as a smooth concave curve between t₁ and t₂
• indicate the new equilibrium concentration of ethanol was higher than the original
• draw a straight, horizontal line to represent the new equilibrium, using a ruler.

Areas for students to improve include:

• taking care to show lines are vertical, horizontal, or curved
• ensuring changes on the graph are located accurately according to the stimulus information
• ensuring the decreasing ethanol curve maintains a consistent concave shape, starting with the highest gradient immediately after t₁ and gradually flattening as it approaches the new horizontal equilibrium line
• ensuring the equilibrium line is drawn as a clear, straight horizontal line with no curvature or ambiguity.

Question 28 (a)

In better responses, students were able to:

• draw neat skeletal or full structural formulae.

Areas for students to improve include:

• identifying that the second monomer of the copolymer will have atoms not present in the polymer.

Question 28 (b)

In better responses, students were able to:

• identify the relevant intermolecular forces
• convey that polymer chains were being separated from other polymer chains, rather than monomers from other monomers, or breaking polymer chains apart
• refer to the properties stated in the question.

Areas for students to improve include:

• engaging with the stimulus material provided to identify relevant structural features.

Question 29

In better responses, students were able to:

• identify that the system remains at equilibrium
• explain with respect to the reaction quotient and equilibrium constant, rates, or Le Chatelier’s principle (LCP).

Areas for students to improve include:

• applying LCP only once to a given system
• recognising that temperature is only directly proportional to pressure if volume and amount of substance are kept constant.

Question 30 (a)

In better responses, students were able to:

• state a specific safety precaution and clearly link it to how the precaution prevents or minimises the associated risk
• recognise the gaseous nature of phosgene was key to its risk of being inhaled, and identify a safety precaution to address that, for example, using a mask or fume hood.

Areas for students to improve include:

• referring to specific safety precautions, such as personal protective equipment (PPE), and the justifications for their use
• using correct terminology for naming safety equipment, for example, fume hood, rather than gas cabinet
• focusing on safety precautions related to people or the environment.

Question 30 (b)

In better responses, students were able to:

• provide a succinct explanation by linking each factor, that is, excess carbon monoxide and catalyst, to its underlying effect
• provide clear, logical reasoning, rather than simply stating the effect
• link LCP for excess carbon monoxide and provide rates of reaction lowering activation energy for catalysts
• link an industrial context of achieving an efficient/cost effective process with maximum yield.

Areas for students to improve include:

• showing the cause and effect relationship, such as by shifting equilibrium to the right, an increased yield results
• providing the effect of catalyst to the rate of reaction, such as saving time/energy/costs rather than simply stating ‘speeds up’ or ‘goes faster’
• recognising an excess reactant in an equilibrium will have an effect on the equilibrium.

Question 31 (a)

In better responses, students were able to:

• construct a correctly balanced equation using the information supplied in the stimulus
• articulate how the balanced equation supports the formula, typically by totalling the number of atoms of N and H on each side of the equation.

Areas for students to improve include:

• constructing the correct formulae for oxygen, nitrogen dioxide and water
• using a balanced equation to work backwards to confirm a chemical formula
• addressing the requirements of the question by providing reasoning beyond a balanced equation.

Question 31 (b)

In better responses, students were able to:

• construct the correct equation demonstrating the acid/base equilibrium, including a reversible arrow (⇌, ⇄, ↔)
• demonstrate the acidic behaviour of the hydrazinium ion (N₂H₅⁺) in water, using N₂H₅⁺ as a reactant and N₂H₄ as a product
• locate the K_w value from the data sheet and use it to resolve for K_a
• resolve for [H⁺] using a correctly rearranged K_a equilibrium expression
• show their calculation steps clearly and with correct values.

Areas for students to improve include:

• addressing the full requirements of the question by including a chemical equation
• using the equilibrium arrow in their equation
• ensuring that chemical equations have correctly balanced charges
• matching the data supplied to the correct species.

Question 32

In better responses, students were able to:

• identify which of the 2 precipitates was most insoluble based on the K_sp values and which precipitate would preferentially form
• write reaction quotient expressions for the precipitation equilibrium and substitute in concentrations successfully
• realise the first precipitate formed will use up some of the magnesium ions so there are less available for the more soluble precipitate.

Areas for students to improve include:

• recognising common insoluble salts using solubility rules
• understanding scientific notation, such as recognising that 10^-7 is larger than 10^-12
• understanding reactions involving limiting reagents
• understanding the difference between molar solubility and ionic product, to test for precipitate.

Question 33

In better responses, students were able to:

• ensure chemical equations were balanced, with the correct states and an irreversible arrow
• disregard the initial titre and use the averaged titre, with all the working out clearly shown and notated, to demonstrate the thinking process as they worked through the complex, multi-step calculation
• identify there were five aliquots and use this to determine the total moles
• write down the result from their calculation and compare this to the brands supplied, rather than simply writing the closest percentage as the calculated value.

Areas for students to improve include:

• understanding equations associated with back titration, including that metal carbonate and acid react to produce a salt, water and carbon dioxide
• including all working in a clear and logical manner, crossing out clearly any mistakes
• starting with the simple steps: equation, moles of the chemicals where volume and concentration data is supplied, mole ratio, average volume from the titres excluding any outlier.

Question 34 (a)

In better responses, students were able to:

• show logical working supported by annotations on the graph, communicating the correct volume at equivalence
• use the equivalence volume to determine the half equivalence point for pK_a calculations
• include the decrease in the original acid concentration. Factors in 0.24 − 10^-2.5 as original [acid]
• equate equivalence point to 0.024 mL and use this to work out half-equivalence of 0.012 mL, and then use this data with K_a = 10^-pKa
• recognise pH = pK_a at half equivalence volume.

Areas for students to improve include:

• annotating the graph correctly to communicate equivalence or half equivalence points
• reading the axes measurement on the graph and using a ruler to draw straight lines for interpolation
• using c=n/v to calculate concentration, and write the expression for K_a, then substitute values, and show all working
• identifying that the pH at half equivalence volume = pK_a
• factoring in the reduction (-10^-2.5) in the concentration of the original [HA] (0.24).

Question 34 (b)

In better responses, students were able to:

• provide valid explanations relating to both decreasing concentration (and pH) due to neutralisation and dilution due to the initial aliquot
• use pH calculations to support their response
• calculate how much NaOH was diluted by the acid
• explain the neutralisation reaction occurring produced water, and pH would not reach 13 due to dilution of the added aliquot of acid.

Areas for students to improve include:

• recognising the system was not a buffer system
• explaining with accurate terminology that neutralisation reactions would decrease the hydroxide ion concentration, and identifying that neutralisation and dilution are reasons for lower pH
• identifying that after the equivalence point, strong base is being added to a salt solution
• using the language of cause and effect.

Question 35

In better responses, students were able to:

• explain how the heat released in an exothermic reaction helped to minimise the disturbance/change and establish equilibrium
• explain that a decrease in temperature slows the rate of both the forward and reverse reactions
• relate the difference in E_a to why the forward exothermic reaction is favoured and state that the forward rate is faster/greater than the reverse rate
• explain that the decrease in temperature favours the forward reaction, increasing the [Co(H₂0)₆²⁺] and making the solution pink.

Areas for students to improve include:

• including an explanation of collision theory that integrates an energy profile diagram
• understanding the difference between an energy profile diagram and an equilibrium graph
• understanding activation energy is not changed by temperature.

Question 36

In better responses, students were able to:

• interpret the skeletal structure accurately to identify the compound as propanoic acid
• describe the predicted spectra for all four techniques with clearly set out, concise responses
• use an annotated structural formula to make explicit links between parts of the structure and the features on the spectra using relevant spectroscopic data from both, the stimulus and the data sheet
• show only data for the relevant parts of the propanoic acid with no erroneous information
• show an extensive understanding of the relevant features of proton NMR spectroscopy, including chemical shifts, integration and multiplicity.

Areas for students to improve in:

• translating between skeletal structures and full structural formulae (especially the COOH functional group), rather than misinterpreting the compound as butanoic or ethanoic acid or a ketone/alcohol
• interpreting the ¹³C NMR data related to carbons adjacent to the carbonyl group, as this was often missed
• referring to the peaks in ¹³C NMR and ¹H NMR being related to the carbon and hydrogens respectively, rather than the bonds between carbons or hydrogens, including multiplicity for ¹H
• recognising which IR peaks are relevant and why a broad COOH with the carbonyl differs slightly to a broad OH peak
• calculating molar mass and relating it to the molecular ion peak, which at times was due to misinterpretation of the skeletal structure or due to an extra hydrogen included in the structure.

Question 37

In better responses, students were able to:

• show a clear logical progression of ideas and correctly interpret each piece of the data given to justify their answer
• address all the stimulus provided and clearly identify the types of reactions occurring
• link multiple concepts to form a succinct and concise extended response, including correct structural diagrams and relevant chemical equations.

Areas for students to improve include:

• drawing organic molecules accurately and demonstrating an understanding of the chemical reactivity of different functional groups
• ensuring all stimulus in the question is addressed and clearly linking the data to their proposed structure
• understanding the difference between primary, secondary and tertiary alcohols and linking the structure to the reactivity of these.